Quantifying the impact of test-trace-isolate-quarantine (TTIQ) strategies on COVID-19 transmission

Ashcroft, P.; Lehtinen, S.; Bonhoeffer, S.

The test-trace-isolate-quarantine (TTIQ) strategy is used to break chains of transmission during a disease outbreak. Confirmed-positive pathogen carriers are isolated from the community to prevent onward transmission and their recent close contacts are identified and pre-emptively quarantined. TTIQ, along with mask wearing and social distancing, make up the non-pharmaceutical interventions that are utilised to suppress the ongoing SARS-CoV-2 pandemic. The efficacy of the TTIQ strategy depends on the probability of isolating a case, the fraction of contacts quarantined, and the delays in these processes. Here we use empirical distributions of the timing of SARS-CoV-2 transmission to quantify how these parameters individually contribute to the reduction of onwards infection. We show that finding and isolating index cases, and doing so with minimal delay after symptom onset, have the largest effects on case reduction, and that contact tracing can make up for deficiencies in testing coverage and delays. These results can be used to assess how TTIQ can be improved and optimised. We provide an online application to assess the efficacy as a function of these parameters.